(1) Field of the Invention
The present invention pertains to a photographic objective lens suitable for use in single-lens reflex cameras, electronic still cameras, and the like, and pertains to a large-relative-aperture internal focusing supertelephoto lens wherein focal length is about 400 mm or greater and F-number is less than about 3.
(2) Background of the Related Art
What is herein referred to as internal focusing is art allowing a photographer to change shooting distance at will (i.e., to focus) from an infinite shooting distance state to a short shooting distance state, without causing a change in the overall length of the lens system, as a result of causing a portion of the lens groups within the lens system to move along the optical axis.
In this type of supertelephoto lens, employed in single-lens reflex cameras, electronic still cameras, and the like, the focusing lens group(s), which move along the optical axis during focusing, have conventionally had large effective diameters.
For this reason, because it has not been possible with conventional supertelephoto lens art to bring down to the same order as that of small-aperture telephoto lenses the effective diameter of the focusing lens group(s) in large-relative-aperture internal focusing supertelephoto lenses wherein focal length is about 400 mm or greater and F-number is about 3 or less, exploitation of commonality between small-aperture telephoto lenses and large-relative-aperture internal focusing supertelephoto lenses has not been permitted with respect to the actuator(s) that drive the focusing lens group(s) and with respect to the hardware structure connecting the actuator(s) and the focusing lens group(s), compelling drastic increases in manufacturing cost. Furthermore, attainment of satisfactory optical performance has also not been permitted.
While Japanese Laid-Open Patent Application (Kokai) No. H8[1996]-327897 is an example of background art directed toward an object similar to that of the present invention, the proposal of this reference is inadequate for purposes of adequately reducing the effective diameter of the focusing lens group(s) while at the same time permitting both long focal length and large relative aperture as does the present invention.
The present invention was conceived to solve such problems as these, its object being to provide a large-relative-aperture internal focusing supertelephoto lens having excellent imaging performance and permitting use of hardware and actuator(s) associated with focusing lens group(s) in cannon with those used in small-aperture telephoto lenses as a result of keeping the effective diameter of the focusing lens group(s) to the same order as that of small-aperture telephoto lenses while maintaining satisfactory optical performance.